Oh my goodness! With the cost of supplies changing daily, using a formula like this (cost x 2) means that you are making your work for free! Please check out this article, How to Price Your Jewelry
, and begin making some money for your art!
You should also consider checking out The Definitive Guide to Selling Handcrafted Jewelry, which goes even further in-depth about pricing, selling, and other aspects of the jewelry business.
Answer contributed by Dale "Cougar" Armstrong